There is known a combustion pressure sensor (also called a cylinder pressure sensor) that is mounted to a combustion chamber of an internal combustion engine. A combustion pressure sensor outputs a signal corresponding to a magnitude of a pressure in a combustion chamber. A change of a combustion pressure with respect to a crank angle depends on the combustion state in a combustion chamber. Therefore, a signal outputted by the combustion pressure sensor is received, and the signal is processed, whereby a combustion state parameter expressing the combustion state in a combustion chamber can be obtained. The combustion state parameters include a combustion ratio (a ratio of a mass of burned fuel to a mass of fuel in the combustion chamber) in a predetermined crank angle, and a crank angle at which a combustion ratio becomes a predetermined ratio, for example. In the latter, the crank angle at which the combustion ratio becomes 50% is specially called a combustion gravity center or CA 50. These combustion state parameters are used in control of a combustion state. For example, Patent Literature 1 as follows describes using CA 50 in optimum ignition timing control.
Further, concerning the form of a combustion pressure sensor, various proposals are made. For example, Patent Literature 3 discloses a combustion pressure sensor which is integrated with a glow plug. In the glow-plug-integrated type combustion pressure sensor, a heater rod that is a heating element also functions as a pressure receiving member. That is to say, the heater rod is displaced in accordance with the pressure in the combustion chamber, and the signal corresponding to the displacement is outputted from the combustion pressure sensor. However, deposit easily adheres to a periphery of the heater rod. The adhering deposit becomes resistance at the time of the heater rod being displaced, and changes the displacement characteristic of the heater rod to the pressure. If the displacement characteristic of the heater rod to the pressure changes, the output value from the combustion pressure sensor is displaced in relation to the time when adhesion of deposit does not occur. Therefore, in the art disclosed in Patent Literature 3, the adhering state of deposit is determined based on the signal from the combustion pressure sensor, and when adhesion of deposit is recognized, energization is performed to the glow plug, and deposit is burned and removed.